In a manufacturing process for a substrate with a circuit pattern for a semiconductor device or a liquid crystal display, a defect such as a break, a short circuit, a flaw, or a foreign material affects the performances of the semiconductor device or the liquid crystal display manufactured from the substrate. For this reason, it is important to early detect such a defect. Along with finer circuit patterns, the inspection device, which uses an electron beam and to which the technique of an electron microscope is applied, has been put into practical use, as well as an optical inspection device using reflected light.
Defect detection with the inspection device using such a charged particle beam is performed by capturing images of a region with a repetitive pattern and an adjacent region thereof and comparing the images with each other. Namely, the defect detection is performed based on knowledge that the above circuit pattern has a feature repeating the same pattern. Alternatively, there is used a defect detecting method of storing an image with a defect-free pattern in a device as a reference image and comparing an image to be detected with the reference image. In such a defect detection, pixel(s) different in signal intensity such as brightness are extracted from the captured image by the pixel. A pixel in which signal intensity exceeds a predetermined threshold is taken as a defect candidate and the representative coordinates thereof is obtained. The reason the image is taken as a defect candidate is that noises are superimposed on the image itself due to various reasons and can be detected as a defect. An operator visually views an image having a detect candidate to determine whether the image is a true defect.
As described above, in the defect detection, images to be compared with each other since are subjected to a computing process, an inspection speed of the inspection device is basically rate-determined by a speed at which an image is captured. However, an area where the inspection device using the charged particle beam can image at one time is very small compared with an area of a substrate to be inspected, so that various methods for reducing an inspection time or improving an inspection speed without decreasing inspection accuracy are attempted.
As an example, there has been known a sampling method which reduces the number of scanning stripes for capturing an image in an image pickup process (hereinafter referred to as a swath sampling). For example, Documents 1 or 2, or Non-Patent Document 2 listed below discloses an inspection device with a function for automatically setting the number of scanning stripes to be set in a chip according to the setting value of a sampling ratio by setting a sampling ratio in setting an inspection region. According to the swath sampling, an imaging area on the substrate to be inspected although is reduced in comparison with a general inspection method, provided the imaging area is sampled by a statistically meaningful method, a problem in manufacture of the substrate can be analyzed by analyzing the distribution of the detected defect candidate or the defect candidate in detail.
Patent Document 3 and Non-Patent Document 1 listed below disclose a reference image averaging (RIA) technique in which, since the swath sampling has a relationship of trade-off between a signal-to-noise ration (S/N) and an image capturing speed, a defect determination method is devised to realize a high-speed inspection. However, a more improved device is demanded to detect an image at a high-speed.